[PATCH v4 2/2] kallsyms: Don't leak pointers for unresolved symbols

[Sebastian Andrzej Siewior]

__sprint_symbol() is supposed to resolve the passed address to a symbol
name. If the symbol can not be resolved it will print the actual pointer
that was passed. The pointer policy is to not reveal actual pointer
values.
For post-mortem analysis the pointer in the backtrace shouldn't be
relevant. In BUG(), WARN() and panic() related backtraces the PC
register will contain the actual value.

Add snprintf_default_pointer() as a wrapper around default_pointer() and
use it __sprint_symbol() if the pointer has not been resolved.

Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 include/linux/sprintf.h |  1 +
 kernel/kallsyms.c       |  3 ++-
 lib/vsprintf.c          | 27 +++++++++++++++++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/include/linux/sprintf.h b/include/linux/sprintf.h
index f06f7b7850911..596c73a57371e 100644
--- a/include/linux/sprintf.h
+++ b/include/linux/sprintf.h
@@ -24,6 +24,7 @@ __scanf(2, 0) int vsscanf(const char *, const char *, va_list);
 /* These are for specific cases, do not use without real need */
 extern bool no_hash_pointers;
 void hash_pointers_finalize(bool slub_debug);
+int snprintf_default_pointer(char *buf, unsigned int len, const void *ptr);
 
 /* Used for Rust formatting ('%pA') */
 char *rust_fmt_argument(char *buf, char *end, const void *ptr);
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index aec2f06858afd..5d51c825c1e71 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -483,7 +483,8 @@ static int __sprint_symbol(char *buffer, unsigned long address,
 	len = kallsyms_lookup_buildid(address, &size, &offset, &modname, &buildid,
 				       buffer);
 	if (!len)
-		return sprintf(buffer, "0x%lx", address - symbol_offset);
+		return snprintf_default_pointer(buffer, KSYM_SYMBOL_LEN,
+						(void *)(address - symbol_offset));
 
 	offset -= symbol_offset;
 
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 40565c857913d..b2ca1d195bc9e 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -849,6 +849,33 @@ static char *default_pointer(char *buf, char *end, const void *ptr,
 	return ptr_to_id(buf, end, ptr, spec);
 }
 
+/**
+ * snprintf_default_pointer - Format a pointer and place it in a buffer
+ * @buf: The buffer to place the result into
+ * @len: The size of the buffer, including the trailing null space
+ * @ptr: The pointer to format
+ *
+ * Formats the passed pointer the same way as the %p modifier. Does not write
+ * more than len bytes to buf including the NULL terminator.
+ *
+ * See the vsnprintf() documentation for the return value.
+ */
+int snprintf_default_pointer(char *buf, unsigned int len, const void *ptr)
+{
+	struct printf_spec spec = {
+		.base = 10,
+		.precision = -1,
+		.field_width = -1,
+	};
+	unsigned int w;
+	char *endp;
+
+	endp = default_pointer(buf, buf + len, ptr, spec);
+	w = min_t(unsigned int, len - 1, endp - buf);
+	buf[w] = '\0';
+	return endp - buf;
+}
+
 int kptr_restrict __read_mostly;
 EXPORT_SYMBOL_FOR_MODULES(kptr_restrict, "printf_kunit");
 

-- 
2.55.0